More specifically, the method applies to a budget (or a height interval with respect to the ground) being allocated to a position error of the aircraft, generated by a positioning system.
This invention more particularly applies, although not exclusively, to a method using, for the navigation of the aircraft, a satellite positioning system, such as the GPS positioning system (Global Positioning System).
An assistance method is known for the navigation of an aircraft flying at a low altitude, using a budget allocated to a position error and a budget allocated to a guiding error. Such budgets are estimated at an integrity risk corresponding to the probability that the value of the real error is higher than the budget allocated to such an error. In such a usual method, the budget allocated to the position error and the budget allocated to the guiding error are calculated for a same given integrity risk being equal to the desired integrity risk for a total budget, such a total budget being equal to the sum of the budget allocated to the guiding error and the budget allocated to the position error.
In such a method, a comparison in real time is made between the variable value of a protection range (calculated at the given integrity risk) and the budget allocated to the position error (estimated at such integrity risk), and if the calculated value of the protection range exceeds the budget allocated to the position error, an alarm is emitted indicating to the pilot that the real position value is likely to be higher than the budget being allocated with a higher probability at the integrity risk being considered. The protection range calculated at the given integrity risk is determined, in real time, as known, by a usual calculation being performed from data from the positioning system and taking into account, more particularly, the geometry of the satellites and the inertia situation of the aircraft.
However, such a usual method does not allow to optimize the size of the total budget, resulting in a problem when it comes to minimizing the height of the aircraft flying over the ground, in particular for tricky steering operations during which the ground controlling means are not available (for example, flies at a low altitude with no visibility, for which only the on-board measurement and calculation means are able to be used).
Indeed, said usual method triggers an alarm as soon as the protection range exceeds the budget allocated to the position error, and the weaker the given integrity risk, the larger the calculated protection range. Now, in order to prevent any accident risk, the integrity risk is selected weak, and the budget allocated to the position error is sized with respect to such an integrity risk. Indeed, as the protection range is large, the budget allocated to the position error is selected large in order to prevent it from being frequently exceeded by the protection range (variable and calculated in real time at the given integrity risk), that would lead to the alarm being frequently triggered, increasing the total budget.
Moreover, the total budget is not optimized, as it is equal to the sum of the budget allocated to the position error and the budget allocated to the guiding error, being estimated at the given integrity risk.